Such a tool can typically be provided on a lower end of a drill string that is deployed in a subterranean bore hole, whereby during operation the drilling fluid is pumped from surface through a longitudinal channel in the drill string to the tool and essentially back to surface in a return stream through an annular space between the drill string and the bore hole wall.
In order to avoid continuous circulation of the abrasive particles through the drill string and the annular space, U.S. Pat. No. 6,510,907 proposes to provide the tool with a recirculation system for separating the abrasive particles from the return stream and re-inserting these particles into a mixing chamber through which the main stream of drilling fluid is passed. Embodiments of the recirculation system are based on a separator magnet for magnetically separating the abrasive particles from the return stream, which abrasive particles are made of a magnetic material for that purpose.
Another such tool is described in International publication WO 02/34653. The recirculation system of that tool is based on a helical separator magnet that is concentrically arranged within a support member. The support member is formed by a cylindrical sleeve, of which sleeve the outer surface forms a support surface on which magnetic particles are retained by the magnetic field generated by the helical separator magnet. The separator magnet has a central longitudinal axis about which the separator magnet is rotatable relative to the sleeve.
When the separator magnet is driven into axial rotation, the magnetic particles experience a moving gradient of magnetic field strength perpendicular to the helical groove, which the particles will follow. In this way the particles are transported over the support surface back to the jetting system for re-insertion into the mixing chamber.
Both prior art tools rely on an efficient transfer of the magnetic abrasive particles from the separator magnet into the mixing chamber for entrainment with the stream of drilling fluid.
In particular when a large number of abrasive particles must be recirculated per time unit, the transfer must be efficient or else the abrasive particle inlet into the mixing chamber can be blocked by an excess of magnetic particles arriving at the inlet.